1. Field of the Invention
This invention relates to a thermoformable, multi-layer plastic sheet with gas barrier properties, the sheet having an inner extrudable gas barrier layer, a glue layer applied to either side of the gas barrier layer, each of the glue layers having a layer of a polyolefin composition bonded to it.
2. The Prior Art
Thermoformable, multi-layer plastic sheets with gas barrier properties are know to the art for the packaging of oxygen and water vapor-sensitive products (British Pat. No. 1,379,106). The plastic sheet is a coextruded sheet, the inner extrudable barrier layer of which consists of a vinylidene chloride/vinyl chloride copolymer, with glue layers of ethylene/vinyl acetate copolymer applied to either side of an outer layer of polyethylene, polypropylene or ethylene/propylene copolymer. Such plastic sheets are suitable for the packaging of products containing volatile flavor which is to be retained to the largest possible extent, as well as for products which may pick up undesirable odors from the environment, or for products containing highly volatile components. Products which may be advantageously packed in such sheets are, for instance, cheese, butter, various medicines, dried food products and shelf-stable meat products.
The known multi-layer sheets of suitable thickness can be converted to comparatively rigid plastic packages using thermoplastic forming techniques. In thermoforming, the multi-layer sheet material is formed under the effect or heat by drawing the sheet web from a reel, holding the lateral edges of the sheet in suitable conveyor means and passing it through a heat-treating zone. The heating can be effected by an oven or by radiant heaters of by contact heating. The sheet so heated is fed directly into a thermoforming machine in which the forming operation is effected, for example by vacuum forming, pressure forming or a similar treatment, in order to adapt the package to be manufactured to the item to be packed. In all cases the sheet material must be heated so that a certain degree of softening is accomplished before it can be converted to the particular shape desired.
A drawback in the thermoforming of multi-layer sheets of the type described above in which the outer layers are formed of a polyolefin composition is the relatively high melt index of this polyolefin composition which, on the other hand, is required for coextrusion with the high density materials of which the gas barrier layers of the type described above are composed. Due to the fact that polyolefins, such as polyethylene, polypropylene or propylene copolymers, have a relatively sharply defined melting point, such sheets, when heated outside the forming station to a temperature adequate for thermoforming, often cannot absorb the tensile forces created by their own weight without noticeable elongation. Therefore the sheets, when passing through the heating station while being supported by the feed chains, very often sag and have to be fed into the forming machine in this condition. Such sagging sheets lead to excessive sheet material being fed to the individual mold cavities in the thermoforming machine, which results in wrinkles or cracks of the packages so manufactured making them unsuitable for the intended purpose.